It is well known in the art to plate non-conductive substrates, (i.e. plastics) with metal for a variety of purposes. Plastic moldings are relatively inexpensive to produce and metal plated plastic is used for many applications. For example, metal plated plastics are used for decoration and for the fabrication of electronic devices. An example of a decorative use includes automobile parts such as trim. Examples of electronic uses include printed circuits, wherein metal plated in a selective pattern comprises the conductors of the printed circuit board, and metal plated plastics used for EMI shielding. ABS resins are the most commonly plated plastics for decorative purposes while phenolic and epoxy resins are the most commonly plated plastics for the fabrication of printed circuit boards.
Plating on plastic surfaces is used in the production of a variety of consumer items. Plastic moldings are relatively inexpensive to produce and plated plastic is used for many applications, including automotive trim. There are many stages involved in the plating of plastic. The first stage involves etching the plastic in order to provide mechanical adhesion of the subsequent metallic coatings and to provide a suitable surface for adsorption of the palladium catalyst which is typically applied in order to catalyze deposition of the initial metallic layer from an autocatalytic nickel or copper plating process. Following this, deposits of copper, nickel and/or chromium may be applied.
The initial etching of the plastic components is an essential part of the overall process. However, only certain types of plastic components are suitable for plating. The most common types of plastic for electroplating are acrylonitrile/butadiene/styrene (ABS) or a blend of ABS with polycarbonate (ABS/PC). ABS consists of two phases. The first phase is a relatively hard phase consisting of an acrylonitrile/styrene copolymer and the second phase is a softer polybutadiene phase.
Currently, this material is etched almost exclusively using a mixture of chromic and sulfuric acids, which is highly effective as an etchant for ABS and ABS/PC. The polybutadiene phase of the plastic contains double bonds in the polymer backbone, which are oxidized by the chromic acid, thus causing complete breakdown and dissolution of the polybutadiene phase exposed at the surface of the plastic which gives an effective etch to the surface of the plastic.
One problem with the traditional chromic acid etching step is that chromic acid is a recognized carcinogen and is increasingly regulated, insisting that wherever possible, the use of chromic acid is replaced with safer alternatives. The use of a chromic acid etchant also has well-known and serious drawbacks, including the toxicity of chromium compounds which makes their disposal difficult, chromic acid residues remaining on the polymer surface that inhibit electroless deposition, and the difficulty of rinsing chromic acid residues from the polymer surface following treatment. Additionally, hot hexavalent chromium sulfuric acid solutions are naturally hazardous to workers. Burns and upper respiratory bleeding are common in workers routinely involved with these chrome etch solutions. Thus, it is very desirable that safer alternatives to acidic chromium etching solutions be developed.
Early attempts to replace the use of chromic acid to etch plastic typically focused on the use of permanganate ions as an alternative to chromic acid. The use of permanganate in combination with acid is described in U.S. Pat. No. 4,610,895 to Tubergen et al., which is herein incorporated by reference in its entirety. Later, the use of permanganate in combination with an ionic palladium activation stage was suggested in U.S. Pat. Pub. No. 2005/019957 to Bengston, which is herein incorporated by reference in its entirety. The use of acid permanganate solutions in combination with perhalo ions (e.g., perchlorate or periodate) was described in U.S. Pat. Pub. No. 2009/0092757 to Satou, which is herein incorporated by reference in its entirety. Finally, the use of permanganate ions in the absence of alkali metal or alkaline earth metal cations was described in International Pub. No. WO 2009/023628 to Enthone, which is herein incorporated by reference in its entirety.
Permanganate solutions are also described in U.S. Pat. No. 3,625,758 to Stahl et al., which is herein incorporated by reference in its entirety. Stahl suggests the suitability of either a chrome and sulfuric acid bath or a permanganate solution for preparing the surface. In addition, U.S. Pat. No. 4,948,630 to Courduvelis et al., which is herein incorporated by reference in its entirety, describes a hot alkaline permanganate solution that also contains a material, such as sodium hypochlorite, that has an oxidation potential higher than the oxidation potential of the permanganate solution and is capable of oxidizing manganate ions to permanganate ions. U.S. Pat. No. 5,648,125 to Cane, which is herein incorporated by reference in its entirety, describes the use of an alkaline permanganate solution comprising potassium permanganate and sodium hydroxide, wherein the permanganate solution is maintained at an elevated temperature, i.e., between about 165° F. and 200° F. U.S. Pat. No. 4,042,729 to Polichette et al., which is herein incorporated by reference in its entirety, describes an etching solution that comprises water, permanganate ion, and manganate ion, wherein the molar ratio of manganate ion to permanganate ion is controlled and the pH of the solution is maintained at 11-13.
As is readily seen, many etching solutions have been suggested as a replacement for chromic acid in processes for preparing non-conductive substrates for metallization. However, none of these processes have proven satisfactory for various economic, performance and/or environmental reasons and thus none of these processes have achieved commercial success or been accepted by the industry as a suitable replacement for chromic acid etching. In addition, the stability of the etching solutions may also be poor, resulting in the formation of manganese dioxide sludge.
The tendency for permanganate based solutions to form sludge and undergo self-decomposition has been noted by the inventors here. Under strongly acidic conditions, permanganate ions can react with hydrogen ions to produce manganese(II) ions and water according to the following reaction:4MnO4−+12-H+→4Mn2++6H2O+5O2  (1)
The manganese(II) ions formed by this reaction can then undergo further reaction with permanganate ions forming a sludge of manganese dioxide according to the following reaction:2MnO4−+2H2O+3Mn2+→5MnO2+4H+  (2)
Thus formulations based on strongly acidic permanganate solutions are intrinsically unstable irrespective of whether the permanganate ion is added by alkali metal salts of permanganate or is electrochemically generated in situ. In comparison to the currently used chromic acid etches, the poor chemical stability of acidic permanganate renders it effectively useless for large scale commercial application. Alkaline permanganate etches are more stable, and are widely used in the printed circuit board industry for etching epoxy based printed circuit boards, but alkaline permanganate is not an effective etchant for plastics such as ABS or ABS/PC. Thus, manganese(VII) is unlikely to gain widespread commercial acceptance as an etchant for these materials.
Attempts to etch ABS without the use of chromic acid have include the use of electrochemically generated silver (II) or cobalt (III). Certain metals can be anodically oxidized to oxidation states which are highly oxidizing. For example, manganese(II) can be oxidized to permanganate (manganese VI), cobalt can be oxidized from cobalt (II) to cobalt (III) and silver can be oxidized from silver (I) to silver (II).
There is currently no suitable commercially successful etchant for plastics based on either permanganate (in either acid or alkaline form), on manganese in any other oxidation state or by using other acids or oxidants.
Thus, there remains a need in the art for an improved etchant for preparing plastic substrates for subsequent electroplating that does not contain chromic acid and that is commercially acceptable.